How genes are differentially regulated in response to environmental and physiological changes is a central question in molecular biology. The process of nitrate assimilation in plants exemplifies a tightly regulated pathway that is sensitive to several environmental factors, the most important being the availability of nitrate and light. The pathway plays a key role in the physiology of plants because nitrate is the primary source of nitrogen for plants, its assimilation is a major energy consuming process, and the pathway is integrally involved with other important metabolic processes such as photosynthesis, pH maintenance, and metabolite transport and storage. The long term objective of this work is to understand how the expression of genes involved in nitrate assimilation is regulated and coordinated in a multicellular organism that uses light as its primary source of energy. The initial focus of the study will be on the first and rate-limiting step in the pathway catalyzed by nitrate reductase. This research aims to describe how nitrate reductase mRNA responds to light and nitrate in the two major organs of plants (leaves and roots), to identify the cis-acting regulatory regions in the gene, to isolate and characterize mutations that alter the regulation of the gene, and to isolate and characterize mutations that alter the regulation of the gene, and to isolate other nitrate-regulated genes. These experiments will be done with Arabidopsis thaliana, a flowering plant with several key advantages for the molecular and genetic analysis of nitrate reductase. The gene for nitrate reductase has been cloned from this plant and has been used to show that the gene is regulated by nitrate at the RNA level. The pathway is conditionally required for growth, and there are several methods for isolating mutants blocked in the pathway including a selection with chlorate. Plants with mutations in the structural gene for nitrate reductase will be examined for regulatory defects to see if nitrate reductase plays a regulatory role in the expression of its gene or of other nitrate-regulated genes.